In the art of planar waveguide circuit substrates, optical waveguides can be formed along with opto-electronic components and/or electrical traces in an effort to reduce signal propagation times across the circuit boards of electronic systems and, in some cases, across integrated circuit chips. As an example, consider an opto-electronic interconnect board intended to electrically connect several integrated-circuit (IC) chips together as a system. There would often be a need in such a system to convey an electrical signal from an IC chip at one end of the board to another IC chip at the opposite end of the board, with a distance of perhaps tens of centimeters between the two IC chips. A conventional electrical signal trace formed in the interconnect board, configured to electrically connect these two IC chips, would have a large amount of resistance and capacitance, and thus would have a large propagation delay. In theory, an optical connection between these two IC chips would not have such a large delay. For such an optical connection, an opto-electric conversion device (e.g., VCSEL or light modulator) may be positioned under one IC chip and electrically connected thereto, another opto-electric conversion device (e.g., photodetector) may be positioned under the other IC chip and electrically connected thereto, and an optical waveguide may be formed between the two opto-electric conversion devices. The optical waveguide comprises a strip of core material, with a square or rectangular transverse cross-section, disposed between two cladding layers.
The performance of such substrate optical transmission systems has not met the full desired expectations, and there is a recognition in the art for improvement of these systems. The distance over which signals can be reliably transmitted is less than desired, and the amount of power required to transmit the optical signal is greater than desired. Current research is focused on improving the opto-electric conversion devices, and their integration onto the substrate. The current view in the art is that improvements will address the less-than-desired performance of these systems.